then press Poly 2. the number of the voice played will now be displayed when you hit a key. This helps figuring out which voice has a problem.

The DCO consists of one Master Oscillator (same for all voices) and two timers (each doing 3 voices). If one voice is out of tune, the problem may be there or in the D/A circuit which provides the CV coming from the keyboard or MIDI.

In my experience, the DCO timing ICs rarely fail, so if it's only one voice that's out of tune, I'd look elsewhere first. Does the out-of-tune voice sound like it has a different timbre than the rest? It could be a VCF that is out of calibration and has way too much resonance.